In the automotive industry, it is known to provide vehicles with radar systems. For example a short range radar system may be provided to aid a driver of the vehicle in determining a distance between the vehicle and nearby objects, such as other vehicles during, say, a parking manoeuvre or the like. Additionally, a longer range radar system may be provided for use with an adaptive cruise control system, whereby the cruise control system maintains a substantially constant distance between the vehicle in which the system is provided and a vehicle in front. Typically, such an automotive radar system comprises a transmitter for transmitting a carrier signal within a given frequency band, for example within a microwave frequency band of between 24 and 77 GHz, and upon which a modulation is applied. The automotive radar system further comprises a receiver for receiving a reflection of the transmitted signal reflected back by a nearby object. A delay and frequency shift between the transmitted and received signals may then be measured, and a distance between the vehicle and the nearby object by which the signal was reflected, and also a speed difference between the two, may be calculated.
The transmitter frequency source used within such automotive radar systems is typically provided by a voltage controlled oscillator (VCO). A problem with the use of VCOs is that their frequency characteristics may vary depending on, by way of example, temperature, supply voltage, manufacturing process variations, etc. Furthermore, the voltage to frequency transfer function of a VCO is not perfectly linear, and the VCO phase noise may be too poor to meet system requirements.